The operation of blast furnaces typically consumes large quantities of air, which provides the oxygen for the oxidation reactions that occur therein. The operators of blast furnaces have been switching to powdered coal injection to reduce the amount of coke necessary for the production of iron from ore. With this change, the air to the blast furnace has to be enriched with oxygen to maintain the blast furnace production rate. This has created a need to produce oxygen-enriched air for use with blast furnaces. The oxygen requirement for such a blast furnace is more than 0.1 ton of oxygen per ton of iron. Consequently, the cost of oxygen becomes an important factor in the cost of producing iron.
Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. Since only the oxygen fraction of the gas injected into the blast furnace (for example, air) is consumed by combustion, the other components of the gas (for example, nitrogen and argon) are usually discharged from the gas furnace chemically unchanged. These unreacted discharge gases, however, have been heated in the process and therefore contribute to the energy inefficiency of the process, since much of this heat cannot be recovered.
The basic means for supplying oxygen-enriched air to the blast furnace involves taking normal purity oxygen (99.5 mol %) from an air separation unit designed for supplying the basic oxygen furnace (BOF) and mixing the gas with the blast air prior to entering the blast furnace stoves. Often the oxygen used for enriching the air is at an elevated pressure needed for BOF (&gt;200 psia) operations.
An alternative method is to use a low purity (70-90 mol %) oxygen plant to produce oxygen using less power than a high purity oxygen plant and mix this oxygen gas with the air from the blast air blower.
Because of this need for oxygen for use in blast furnaces, there is a demand for the development of more efficient processes for producing oxygen or oxygen-enriched air for use in blast furnace operations. The invention meets this demand by integrating gas separation processes to produce oxygen or oxygen-enriched air with the blast furnaces they supply so as to enhance the efficiency of the overall process.
Solid electrolyte ionic conductors offer a potentially attractive technology for the separation of oxygen from air. The solid electrolyte process may be operated using the ionic conductors in an electrically-driven mode or mixed conductors in a pressure-driven mode. Two unique features of the solid electrolyte process are that the process operates at high temperatures (.about.600.degree.-1000.degree. C.) and produces oxygen with an infinite selectivity of oxygen to nitrogen. These features make the solid electrolyte process well suited for integration into a high temperature process such as blast furnace operation.
The basis for the operation of ion transport membrane is that it efficiently transports oxygen ion vacancies at high temperatures. Electrically-driven ion transport membranes, when exposed to differential oxygen partial pressure on both sides of the membrane, will allow a spontaneous voltage (the Nernst potential) to develop that is logarithmically dependent on the oxygen partial pressure across the membrane. Conversely, when an external voltage in excess of the Nernst potential is applied, oxygen in the form of oxide ions can be pumped across the membrane against the partial pressure gradient. This pumping requires an electrical current and, while this type of process is capable of producing oxygen at elevated pressure and temperature, the cost of the electrical energy required is high.
More recently, solid electrolyte materials have been developed that can transport oxygen ion vacancies at high temperature and are also electronic conductors. For such materials, the counter-current to the flow of oxygen ion vacancies is carried by an internal flow of electrons, rather than through an external circuit. No electrodes are required and the entire transport is driven by the oxygen partial pressure in the gas streams on either side of the ion transport membrane. No electrical energy need be supplied and this type of process is more readily integrated with the blast furnace equipment and is a more attractive means for supplying oxygen or oxygen-enriched air for the blast furnace.
There are therefore two types of ion transport membranes in use: ionic conductors that conduct only ions through the membrane, which require electrodes and an external circuit to enable flow of electrons, and mixed conductors that conduct both ions and electrons through the membrane. As used herein, the terms "solid electrolyte ionic conductor", "solid electrolyte ion transport membrane", "solid electrolyte" or "ion transport membrane" are generally used to designate either an ionic-type (electrically-driven) system or a mixed conductor-type (pressure-driven) system unless otherwise specified.
Although the ion transport process is capable of producing pure oxygen, the best mode of practice for this application involves the use of air, or a gas with a lower oxygen partial pressure than the feed gas stream, as a purge gas stream for the permeate side of the ion transport membrane. This reduces the oxygen partial pressure and enhances the oxygen transport through the membrane leading to a greater oxygen recovery. The product from such an ion transport module is oxygen-enriched air, rather than pure oxygen, but this is suitable for injection into the feed or the blast air stream to elevate the oxygen concentration of the hot blast air.
Solid electrolyte ion transport technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art.
Advances in the state of the art of air separation using inorganic oxide membranes have been presented in the technical literature. In addition, schemes have been proposed (for example, Rathbone, U.S. Pat. No. 5,268,019, see below) in which gas turbines that are fueled with blast furnace gas are integrated with air separation units to provide reduced purity oxygen for blast air enrichment.
Hegarty, U.S. Pat. No. 4,545,787, entitled Process for Producing By-Product Oxygen from Turbine Power Generation, relates to a method of generating power from a compressed and heated air stream by removing oxygen from the air stream, combusting a portion of the resultant air stream with a fuel stream, combining the combustion effluent with another portion of the resultant air stream, and expanding the final combustion product through a turbine to generate power. Hegarty mentions the use of silver composite membranes and composite metal oxide solid electrolyte membranes for removing oxygen from the air stream.
Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature Method for Oxygen Production, relates to a process for separating oxygen from heated and compressed air using a solid electrolyte ionic conductor membrane where the nonpermeate product is heated further and passed through a turbine for power generation.
Rathbone, U.S. Pat. No. 5,268,019, entitled Air Separation Method and Apparatus Combined with a Blast Furnace, relates to a means of integrating an air separation plant with a blast furnace. The method does not involve solid electrolytes and, therefore, makes no use of thermal integration.
Rathbone, U.S. Pat. No. 5,317,862, entitled Air Separation, relates to the use of pressurized nitrogen to generate power and improve the heat balance of a process integrated with a blast furnace.
Grenier, U.S. Pat. No. 5,244,489, entitled Process for Supplying a Blast Furnace with Air Enriched in Oxygen, and Corresponding Installation for the Reduction of Iron Ore, relates to a means for integrating a cryogenic air separation plant with a blast furnace. No solid electrolytes are employed and the invention involves the use of the blast air blower in combination with a low purity air separation concept known as the mixing column process. This is a cryogenic process similar to the standard double column, but with the addition of a third column wherein liquid is contacted with air to produce a low purity stream to mix with the blast air entering the stoves.